1. Field of the Invention
The present invention relates to apparatus and methods for converting heat energy into mechanical energy.
2. Description of the Prior Art
The law of conservation of energy states that energy may be transformed from one kind to another, but it cannot be created or destroyed. Further, energy is defined as the ability to do work. Mechanical energy is more convenient for doing work of most kinds, various apparatus for converting heat energy into mechanical energy have been developed. These are generally called “heat engines.”
The steam engine is an engine in which water is superheated to create high pressure steam, that in turn pressurizes a cylinder containing a piston. The pressure causes the displacement of the piston. The axial motion of the piston translates its energy to a crankshaft that rotates as a result of the piston motion. This results in mechanical work.
The steam turbine also utilizes super heated water vapor to generate mechanical work. The high pressure steam in this system applies a force normal to the turbine fins attached to a rotating armature. Hence the applied force results in the armature rotating.
The Stirling engine is a well known heat engine operating in two general modes during a cycle. In the first mode, the expansion cycle heats the internal gas via an external heat source. The gas expands and moves a first piston. In the second mode, the gas is cooled, retracting a second piston. FIG. 1 (prior art) is a simplified block diagram of one type of Stirling engine 100. Like any heat engine it requires a heater 102 and a cooler 104. Engine 100 has two pistons, a hot piston 106 and a cold piston 108 within cylinders 110 and 112 respectively. Cylinders 110, 112 contain a working gas which (like all gases) expands when heated and compresses when cooled. Cylinders 110, 112 are connected a regeneration area 114 which is used to store heat energy during one part of the Stirling cycle and return it to the working gas 116 in another part of the cycle.
According to the ideal gas law, PV=nRT, where P is pressure, V is volume, n is the number of moles of gas, R is a gas constant and T is temperature. So temperature is proportional to pressure time volumes. Hence, when a gas is heated it expands if possible, and otherwise the pressure increases.
The Stirling cycle has four phases, Isothermal Compression, Constant Volume Heating, Isothermal Expansion, and Constant Volume Cooling (these phases are somewhat simplified for this explanation). Isothermal Compression occurs as heat is transferred from the hot gas 116 to a cold sink, and the gas compresses, drawing piston 108 up from its full capacity. In the present case, the heat is removed by cooler 104, perhaps by simply conducting the heat away from the engine. Some heat is also stored in the regenerator 114 (which might be a network of wires or the like).
Once cold piston 108 is in its intermediate position, the Constant Volume Heating phase begins. Cold piston 108 moves up to its minimum capacity position and then hot piston 106 moves down to an intermediate position. Gas 116 hence passes through regenerator 114 and is heated. Since volume remains the same and the temperature of the gas increases, pressure goes up.
In the third phase, Isothermal Expansion, heater 102 heats the gas. It expands and moves hot piston 106 down to its full capacity position. In the Constant Volume Heating phase, hot piston 106 moves up to its minimum capacity position and then cold piston 108 moves down to its minimum capacity position, again passing gas 116 through regenerator 114. Heat is passed from the gas to the regenerator, so its pressure and volume both remain constant.
Practical Stirling engines have been built. For example, some submarines use Stirling engines. A recent example of a Stirling engine is described in U.S. patent application Ser. No. 6,062,023 to Kerwin et al. Known Stirling engines generally require an extremely hot heat source (600 to 800 degrees Celsius) and a temperature gradient of at least 400° C. The gases used in these engines, for example Nitrogen and Carbon Dioxide are in the gas phase at all times. Thus, current Stirling engines operate at impractically high temperatures and do not take advantage of the liquid phase of the working gas.
A need remains in the art for improved heat engines that operate at more practical temperatures, do not require extreme heat gradients, and utilize the liquid phase of the refrigerant.